1. Field of the Invention
The present invention relates to the synthesis of a silica-enriched crystalline aluminosilicate having the offretite structure, the aluminosilicate thus obtained and its use as a catalyst in the conversion of hydrocarbons.
2. Description of the Related Art
Offretite, which was first identified by Professor GONNARD in 1890 in the Mont Simiouse basalts, was not definitively identified and distinguished from erionite until 1967 by BENNET and GARD (Nature 214 1005 (1967)).
Offretite belongs to the chabazite group and crystallizes in the hexagonal system. Its structure is made up of a stack of "cancrinite" frameworks connected by hexagonal prisms. The "cancrinite" frameworks have openings on 6 sides 0.18 nm in diameter. These stacks are connected to one another by "gmelinite" frameworks. These frameworks have openings on 8 sides, accessible to molecules having a critical diameter of about 0.5 nm. The spatial arrangement of these entities leads to a porous structure characterized by channels 0.64 nm in diameter, accessible through 12 tetrahedral openings. Offretite is thus classified amongst the zeolites in which the pore openings are of average size.
Erionite has the same stacks of cancrinite frameworks and prisms, but a rotation of 60.degree. between two successive cancrinite frameworks causes a periodic obstruction of the channels. In some cases there is intergrowth between offretite and erionite, which adds to the difficulty in identification. Powder X-ray diffraction does not enable these two structural types to be distinguished.
However, it is possible to distinguish between these two structures by other methods, such as electron diffraction, X-ray diffraction on a monocrystal, or the measurement of the strain index (SI).
According to this latter method, developed by FRILETTE (J. CATAL. 67,218, (1981)), the relative rates of cracking of n-hexane and 3-methylpentane are compared.
The strain index (SI) is defined as being the ratio of the logarithm of the fraction of unconverted n-hexane to the logarithm of the fraction of unconverted 3-methylpentane. The test is carried out under atmospheric pressure on an equimolecular mixture. It is possible either to observe the change in the strain index as a function of time or to fix a given time, in general 20 min.
In the absence of steric strain, the strain index is low (less than 1). Steric strains associated with the structure (small-pore zeolite) or with ageing (narrowing of the pores due to coking) lead to high values. For example, erionite, because of the periodic obstruction of the channels, gives a value largely greater than 10.
One of the major uses of zeolites is acid catalysis. In fact, zeolites are aluminosilicates composed of chains of SiO.sub.4 and AlO.sub.4 .sup.- tetrahedra. Electrical neutrality is ensured by compensating cations. In general these cations are K.sup.+ or Na.sup.+ ions replaceable by NH.sub.4 .sup.+. A heat treatment enables NH.sub.4 .sup.+ to be converted to H.sup.+ and thus enables acid solids to be obtained.
It is well known that for each reaction catalysed by a zeolite the aluminic molar fraction m, which is defined as the atomic ratio ##EQU1## plays a fundamental role.
It directly determines the density of acid sites. A change in the aluminic fraction causes a change in the number and the strength of acid sites. This change can be obtained during the synthesis or by a post-synthesis treatment. In the case of a post-synthesis treatment, such as a hydrothermal treatment, this change is accompanied by the creation of a mesoporosity, which is particularly significant in the case of zeolites having a unidirectional structure because this mesoporosity enables the channels in the zeolite to be connected. There is thus an optimum aluminic fraction which is a function of the zeolite and of the reaction under consideration.
ZSM-5, a zeolite discovered by Mobil and having a structure of the MFI type and described in the "Atlas of zeolite structure types" by Meier and Olson (1987 edition, Butterworth), is obtained by direct synthesis with a low aluminic fraction of less than 0.1.
Offretite, the structure of which is described in the same atlas, results from the hydrothermal crystallization of an aluminosilicate gel taken in the system comprising a source of silicon, a source of aluminium, an alkali metal, which is generally potassium, and a structuring agent, which is generally a quaternary ammonium ion such as the tetramethylammonium ion. At the end of the synthesis, the aluminic molar fraction is about 0.2.
In the case of offretite, direct synthesis does not enable the aluminic fraction to be changed in a significant manner. The only route then open for changing this ratio is that of modifying the framework already formed. This route is not without risk, the crystalline structure of offretite being very easily destroyed by these treatments. Thus, the dealumination of offretite by acid treatment for a long time appeared to be impossible, the framework being destroyed by these treatments (F. HERNANDEZ, R. IBARRA, F. FAJULA, F. FIGUERAS, Acta Phys. Chem. 31, 81 (1985)).
A process for the dealumination of offretite has recently been developed by the Institut Francais du Petrole (French Petroleum Institute). This process, described in European Patent Application 190949, requires numerous calcination, cation exchange, steam treatment and acid attack steps.
The multiplicity of these elementary steps makes industrial implementation of this process uncertain and in any case extremely expensive.